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Irritations and Allergic Reactions. External assaults to the skin can also cause formation of dark skin patches in underarm area. For example, chemicals in underarm products such as deodorants and antiperspirants are known for causing allergic reactions and irritations of the skin. Another type of external assault to the skin is excessive rubbing.
First, because of the number of lymph nodes in the area, itching can be the result of a lymph node problem. Secondly, when your arm is at your side, your armpit is closed. The area is therefore constantly warm at times and damp as well, making it a breeding ground for bacteria and fungi.